U.S. Pat. No. 3,304,816 discloses a multi-section drill, separable from its driving means, which uses a coupling mechanism of a mating undercut arrangement in the form of a helix on corresponding male and female ends of the sections to coaxially join the sections together, detachably securing a cutting-head (driven section) in a shank (driving section) of the drill. The driving section (shank) comprises a hollow cylindrical sleeve on a driving end thereof; an undercut in the sleeve in coaxial relationship therewith; two non-parallel edge surfaces on the undercut, one edge having a helix thereon, while the other surface is flat throughout its full length and perpendicular to the rotational axis of the tool; and a back taper on the helix. The driven section (cutting-head) comprises a generally cylindrical body, an undercut on the body forming a cylindrical section thereon; an enlarged coaxial flange on the cylindrical section; two non-parallel edge surfaces on the enlarged flange, one edge having a helix thereon, while the other surface is flat throughout its full length and perpendicular to the rotational axis of the tool; and a back taper on the helix. Cooperation of the flange on the cutting-head and the undercut in the shank forms a compound locking and driving means, as the flange is firmly engaged between the two non-parallel surfaces of the driving portion, while the cylindrical surface of the cutting-head is piloted in the hollow sleeve of the driving portion, so that the two flute surfaces on the driving portion and the cutting-head are mutually aligned.
In order to achieve positive lock together with accurate rotational positioning, the mating surfaces of '816 must be ground to exacting tolerances. However, even if these surfaces are initially precisely fitted, deformation during work may cause dimensional variations, and because there is no definite rotational stop to prohibit over rotation, the parts can rotate relative to each other, especially under working loads, thereby losing their rotational alignment, leading to over-tightening, difficulty in removing the tip, and disturbance to the flow of fluid and chips.
Additionally, since the cutting-head is inserted into the driving section, a clearance fit has to exist between the cylindrical surface of the cutting-head and the hollow sleeve of the driving section, thereby preventing precise axial alignment between the two parts.
U.S. Pat. Nos. 3,153,356 and Re. 26,452 disclose a multi-section gun-drill tool in which the tool may be extended by threading sections coaxially together; the threads are helical in form with a specified helix angle range for locking the male and female threads to each other as the means for coaxially aligning the tool sections. A front shoulder of the cutting-head cooperates with an abutment on the shank, with the tolerances between these two surfaces being exceedingly important, since they limit the turning of the cutting-head threads into the threaded counter-bore of the shank. However, lack of positive stopping and torque-transmitting means can cause the problems mentioned above, i.e., loss of rotational alignment, over-tightening, difficulty in removing the tip, and disturbance to the flow of fluid and chips. Additionally, since axial alignment of the cutting-head and the shank relies on the exacting tolerances between the shoulder of the cutting-head and the abutment on the shank, precise axial alignment between the two parts is difficult to achieve.
Another example of detachable tip gun-drill is disclosed in U.S. Pat. No. 2,346,546 which shows an arrangement for extending the length of the tool by slip fitting a formed male section into a correspondingly sized and shaped female section. Positive torque-transmission and rotational alignment are achieved through a cross-pin which is inserted transversely to the axis of rotation of the gun-drill, into cross-bores passing through the tip and the shank. However, cutting-heads (or tips) made of exceptionally hard materials, such as ceramics or sintered hard metal composition, are difficult and expensive to cross-drill. Additionally, such pins complicate installation and replacement of the cutting-head in the shank.
U.S. Pat. No. 5,971,673 discloses a twist drill having a detachable cutting tip, the drill having fluid conduits extending through the tool body and the cutting tip. To connect the cutting portion to the tool body, the cutting portion and tool body are converged longitudinally so that the projections enter the front flutes. Then, the cutting portion is rotated relatively to the tool body, to align the front flutes with the rear flutes while causing the projections to enter the recesses and form therewith a bayonet, with a support surface of the cutting tip abutting a front surface of the tool body. However, the drill disclosed in '673 cannot provide sufficient axial and lateral support required by gun-drills, where the cutting tip must support and guide the shank, aligning it both axially and laterally, to assure drilling a straight bore. Furthermore, the connection method disclosed in '673 cannot be applied to cutting-tools having a single flute.
It is an object of the present invention to provide a single fluted gun-drill having a detachably secured cutting-head that significantly reduces or overcomes the aforementioned disadvantages, while facilitating quick and simple replacement of the cutting-head.